Ultrasound imaging systems have become a well-accepted and important modality of diagnosis and guidance in many health care fields. For example, fetal monitoring, abdominal soft tissue study, and cardiac monitoring have all incorporated ultrasound systems as an essential aspect of effective diagnosis and treatment. Real time systems, wherein organ and organism motion and development is observed as it occurs, has allowed practitioners to review many physiological conditions in vivo, in substitution for traumatic exploratory surgery, or, worse still, for essential uncertainty as to the nature of a patient's condition.
In accordance with the knowledge of those of ordinary skill in the art, real time scanning systems work in a number of ways, including scanning an area of tissue by physical movement of an ultrasound transducer. In some systems, the transducer is coupled directly to the body of the patient, whereas in others the transducer is spatially separated from the body of the patient by a sonically conductive water path. In either case, as the transducer is "wobbled," typically by a stepping motor, the transducer is alternatively conditioned to transmit a pulse of sonic energy into a tissue region, and then to receive echoes resulting from passage of the pulse through various tissue interfaces. Electronic signal processing and display apparatus assembles information resulting from the echoes, and based on the transducer position and focal conditions, and upon the relative timing of the pulse transmission and echo receipts, a representation or image of the irradiated tissue is assembled. For real time systems, which require a high frame repetition rate, it is vital that pulse transmission and echo receipt be carefully timed and coordinated with respect to the transducer positioning, and further that all such transmission and reception information be well coordinated with the sequential rotational displacement of the transducer itself.
Recently, moving transducers have been applied to other sorts of ultrasound diagnostic systems, including water path systems useful for screening specific organs, such as the breast, for malignancies. In accordance with such systems, the patient is conveniently positioned with the breast downwardly suspended in a tank of water, and from beneath, an oscillating or "nodding" transducer is scanned across the breast area, yielding a succession of spaced apart "B" scan images. In the aggregate, these scans depict substantially all tissue within the breast, subject only to the limits of resolution of the system with respect to each scan, and the spacings of the separate scans. While such screening systems may or may not be utilized by the practitioner as real time systems, the continuous, serial accumulation of data, in a rapid fashion to assemble a significant number of frames of information in a short time, imposes similar timing and signal processing constraints as are in effect in real time imaging of moving organs or organisms. Clearly, failure accurately to correlate transmit data with received data, and in turn with transducer positioning, will completely obviate the effectiveness of scanning for small (e.g. 1-3 millimeters) lesions, either by improperly locating them, or by losing the critical data altogether.
One prior art approach to locating accurately the position of an oscillating transducer has been utilization of a rotational variable displacement transformer (RVDT), mounted to the sonic transducer shaft, at all times, to determine the angular position of the transducer and in turn of the transmitted beam. Modulated carrier signals generated by the RVDT typically are digitized by an analog to digital converter, with the digital signal being used to signify angular position of the transducer during an electronic construction of a B-scan image from A-scans taken at discrete transducer positions. Under optimum mechanical and environmental conditions, the RVDT approach yields adequate transducer monitoring and control capacity, but unfortunately such systems have proven to be susceptible to mechanical and environmental difficulties, requiring frequent and difficult maintenance checks and electronic or mechanical corrections.
It is a primary object of the present invention to provide methods and apparatus for accurately monitoring the position of an oscillating, transmitting and receiving sonic transducer in ultra-sound imaging and diagnostic systems. It is an associated object to provide for such monitoring in a fashion which adequately insures the generation of monitoring and control data whereby electronic image reconstruction apparatus functions accurately and rapidly. It is a further object to provide oscillating transducer monitoring and control apparatus and methods which are mechanically and electrically simple and reliable, involving a relative minimum of sensitivity to unavoidable environmental or mechanical wear constraints.